Distance measuring technique using an ultrasonic wave propagative in the atmosphere and/or a liquid or an elastic wave propagative in a solid is conventionally known, an example of which is disclosed in Japanese Unexamined Patent Publication.
Specifically, such distance measuring technique can measure the distance of a point from which the waves have been transmitted to a target from which they are reflected based on a propagation delay time of the waves between the point and the target.
A distance measuring device designed to use the technique is commonly configured to generate, at an arbitrary generation time, a tone burst signal composed of a continuous wave train consisting of several cycles of a constant frequency over a constant period. The distance measuring device is commonly provided with a detector located in a propagation path of the tone burst signal. The detector is configured to detect the propagating tone burst signal.
The distance measuring device is also commonly configured to:
monitor the level of the tone burst signal being detected by the detector;
determine a detection time at which the monitored level reaches a predetermined level;
calculate, as a propagation delay time, the difference between the generation time and the detection time; and
obtain a propagation distance of the tone burst signal by multiplying the propagation delay time by a propagation velocity of the tone burst signal.
In such a distance measuring device, the detected tone burst signal is digitized to downsize the device and/or to enhance the performance of the device. When the detected tone burst signal is sampled at a predetermined period and converted to an equivalent digital signal, the resolution of the detection signal and therefore the distance to be measured based on the tone burst signal are defined by the sampling period of the tone burst signal.
Specifically, in the distance measuring device designed to measure a distance based on a detected tone burst signal in the form of digital data, the sampling period will have to be shorter, in other words, the sampling frequency will have to be higher in order to improve the resolution of the distance to be measured. This may require an A/D converter and the like, which are operable at a high sampling frequency, causing the distance measuring device to increase in complexity and in cost.
In addition, in the above-mentioned method of determining the detection time based on the monitored level of the detected tone burst signal, the accuracy of determining the detection time may be susceptible to the strength of the propagating tone burst signal, in other words, the level of the detected tone burst signal and/or noise superimposed on the detected tone burst signal. This may reduce the reliability of measurement results of the distance measuring device.
The reasons for the reduction of the reliability are probably as follows.
Specifically, if the tone burst signal has not reached the detector yet, but a noise signal with large amplitude enters the detector, the detector would erroneously determine that the tone burst signal has already reached the detector upon entry of the noise signal.
In addition, if the tone burst signal has already reached the detector, but a noise signal out of phase with the tone burst signal enters the detector so that it reduces the level thereof, the detector would also erroneously determine that the tone burst signal has not reached the detector yet.